1. Field of the Invention
This invention relates to fuel cell systems with at least one fuel cell stack and an external manifold and, in particular, to a seal for use in a fuel cell system having at least one externally manifolded fuel cell stack. More particularly, the invention comprises a caulk member between a manifold and fuel cell stack that minimizes gas leakage, maintains electrical isolation and inhibits electrolyte migration.
2. Description of the Related Art
A fuel cell is a device that directly converts chemical energy in the form of a fuel into electrical energy by way of an electrochemical reaction. In general, like a battery, a fuel cell includes a negative electrode or anode and a positive electrode or cathode separated by an electrolyte that serves to conduct electrically charged ions between them. In contrast to a battery, however, a fuel cell will continue to produce electric power as long as fuel and oxidant are supplied to the anode and cathode, respectively.
In order to produce a useful amount of power, individual fuel cells are typically arranged in stacked relationship in series with an electrically conductive separator plate between each cell. A fuel cell stack may be categorized as an internally manifolded stack or an externally manifolded stack. In an internally manifolded stack, gas passages for delivering fuel and oxidant are built into the fuel cell plates themselves. An internal manifold design requires no external manifold seal, but is expensive to manufacture.
FIG. 1 is an exploded view of an externally manifolded fuel cell stack. As shown in FIG. 1, the individual fuel cells 7 are left open on their ends and gas is delivered by way of manifolds or pans 1 sealed to the perimeter of the respective faces of the fuel cell stack 6. The manifolds provide sealed passages for delivering fuel and oxidant gases to the fuel cells 7 and preventing those gases from leaking either to the environment or to the other manifolds (not shown in FIG. 1). The manifolds must perform their functions under the conditions required for operation of the fuel cell stack and for the duration of its life.
The performance of an externally manifolded fuel cell stack depends in large part on the seal established between the manifold edge and the stack face. The typical design of a manifold seal assembly with external manifolds is shown and described in U.S. Pat. No. 4,467,018. The manifolds, which are constructed from metal, must be electrically isolated from the stack face, which is typically electrically conductive and has an electrical potential gradient along its length. Dielectric insulators, such as the dielectric frame 4 shown in FIG. 1, are used between the metallic manifold and the fuel cell stack to electrically isolate the manifold from the stack and to prevent the manifolds from shorting the stack. Dielectric insulators are typically made from brittle ceramic materials such as alumina and mica, which are rigid and may be easily damaged by thermal and mechanical stresses applied on the manifold system during fuel cell stack operation.
In order to withstand the stresses imparted on the manifold system during operation of the fuel cell stack while maintaining electrical isolation between the manifold and the stack, improvements have been made to the manifolds and to the dielectric insulators used to isolate them from the stack. For example, in the flexible manifold system described in U.S. Pat. No. 6,887,611, the manifolds conform to changes in stack shape and size. A common dielectric insulator assembly is designed as a rectangular frame with joints that allow for differential movement between the stack and manifold. Such a construction is shown and described in U.S. Pat. No. 4,414,294, which discloses a rectangular insulator frame having a plurality of segments interconnected by slidable spline joints that permit expansion or contraction with the walls of the manifold and the fuel cell stack. An alternate dielectric design is described in U.S. Pat. No. 6,531,237, which describes a manifold and manifold sealing assembly having a plurality of dielectric frame assemblies. High-density and highly polished ceramics such as those described in U.S. Pat. No. 6,514,636 are desirable for use in dielectric insulators, to provide the required voltage isolation by preventing or reducing electrolyte creep over the surface of the dielectric frame.
Manifold compression against the stack face and stack compaction during operation cause mechanical stresses which are not completely accommodated by the ceramic dielectrics and may still damage them. Accordingly, various improvements have been made to the manifold-stack seal to better accommodate thermal and mechanical stresses, as well as improve the ability of the seal to reduce gas leakage. For example, a compressible ceramic felt gasket placed between the dielectric insulators and the stack edge (e.g., gasket 5 in FIG. 1) may contain an embedded compliant member that accommodates growth of bipolar plates over time during operation of the fuel cell stack and conforms the gasket to the dielectric joints, as described in U.S. patent application Ser. No. 10/627,035 filed Jul. 25, 2003, and assigned to the same assignee hereof.
However, due to the limitations of gasket materials and the non-uniform stack edge against which the gaskets are placed, the external manifold-stack seals presently used in the art are still not completely effective in eliminating gas leakage between the external manifolds and the stack face. More particularly, as shown in FIG. 2, which is a schematic side view of a portion of a fuel cell stack, each fuel cell in the stack has a cathode and anode (both generally represented by electrode 9 in FIG. 2), and an electrically conductive separator plate 10. Various designs of separator plates have been disclosed, such as in U.S. Pat. No. 4,514,475, which teaches a separator plate that can adjust to changes in thickness of cell parts during operation of the stack; U.S. Pat. No. 5,399,438, which teaches a stainless steel member with high corrosion resistance; U.S. Pat. No. 5,773,161, which teaches an improved bipolar separator structure that assists in electrolyte management by providing trough areas for dispersal or absorption of electrolyte; and U.S. Pat. No. 6,372,374, which teaches a bipolar separator plate with two pairs of opposing pocket members that are welded to a stainless steel plate member. Each cell also includes corrugated current collectors 11, as described for example in U.S. Pat. No. 6,492,045, and an electrolyte matrix 12. The fuel cells are stacked in series with a bipolar separator plate 10 between each cell.
As known in the art, the three-dimensional S-shaped structure of the bipolar plate is formed by welding the pieces that form top and bottom troughs of the separator plate to the edges of the center plate. When the edges are welded and the separator plate is folded and bended, the welded edge has a radius, which is referred to as a weld bead 13. Thus, as can be seen in FIG. 2, although the individual fuel cells provide solid edges against which a manifold gasket may be compressed, the edges of the fuel cell stack do not provide a smooth surface. Even with improved materials and structural features to accommodate growth of bipolar plates and changes in the stack shape, gaskets presently used in the art cannot form a completely effective seal when positioned against the rough surface formed by the stacked cells.
Another limitation of the seal or gasket presently used between the dielectric insulators and the edge of the stack face is that it generally has a small pore size such that it permits electrolyte to be easily absorbed during operation of the stack, which may cause undesirable transport of electrolyte from the top or positive end of the stack to the bottom or negative end. If unchecked, such vertical electrolyte migration can deplete cells of electrolyte at the positive end of the stack and cause the fuel cells at the negative end of the stack to flood. Another type of harmful electrolyte migration that can occur is movement of electrolyte from the stack across the dielectric and to the manifold, which can short the stack. Electrolyte migration is a significant factor in reducing the efficiency and shortening the life of a fuel cell stack.
Methods and devices for reducing or mitigating electrolyte migration in fuel cell systems have been discussed in U.S. Pat. No. 4,643,954, which teaches a passageway along the height of a fuel cell stack with electrolyte-wettable wicking material at opposite ends thereof, equalizing molten electrolyte content throughout the stack; U.S. Pat. No. 4,761,348, which teaches a fuel cell stack having a combination of inactive electrolyte reservoirs at the upper and lower end portions that mitigate the ill effects of electrolyte migration, and a porous sealing member with low electrolyte retention that limits electrolyte migration; and U.S. Pat. No. 5,110,692, which teaches a manifold gasket for molten carbonate fuel cells having an elongated porous member that supports electrolyte flow and barrier means for retarding such flow, which together control electrolyte flow and reduce electrolyte migration. None of these improvements, however, also provides a more efficient gas seal between the manifold gasket and stack face.
Therefore, there is a need for a manifold-stack seal that reduces or eliminates electrolyte migration, while providing an improved gas seal and maintaining electrical isolation of the manifold from the stack.
Another consideration is that fuel cells operate at very high temperatures. For example, molten carbonate fuel cells operate at about 650° Celsius. The selection of materials to be used in any manifold-stack seal must account for this long term operating temperature and allow the components to last for the life of the fuel cell stack, which is typically several years.
Accordingly, there is also a need for a manifold-stack seal that tolerates fuel cell stack operating temperatures and can accommodate stack movement and changes in stack dimensions.
It is therefore an object of the invention to provide a fuel cell manifold-stack seal for sealing a manifold to the face of a molten carbonate fuel cell stack that provides an improved gas seal between the manifold and stack and keeps the manifold electrically isolated from the stack, and also accommodates differential movements resulting from thermal stresses and internal fuel cell compactions during operation of the fuel cell stack.
It is a further object of the invention to provide a manifold-stack sealing assembly that inhibits both electrolyte migration from the positive end of the stack to the negative end, and electrolyte migration from the stack across the dielectric to the manifold.